Cells can have a transmembrane voltage that can be used to facilitate electro-chemical transport of molecules and ions across its membrane. At resting values and measured across the membrane, this voltage can be negative, and can have a small magnitude in the order of a few 10's of mV. As shown in FIGS. 1A, 1B and 2, the transmembrane voltage can vary based on the cell type, function and cellular activity. The transmembrane voltage can also vary in response to a voltage imposed from an external source. The membrane potential 205 can rise or fall by few 10's of mV during normal cellular processes. The membrane potential can also be altered or returned to resting values by the cell through the coordinated action of enzymes such as Na+/K+-ATPase (“ATP”), ion pumps and voltage activated channels. It is previously known that the physiological failure of the enzyme or ion pumps can lead to ionic imbalance and changes in osmolarity, which in extreme cases can lead to cell death. (See, e.g., FIGS. 1A-1D, 2, 3 and 4). The exemplary voltage (e.g., an external voltage 210) imposed upon the cell generated from an external source can be considered an external voltage, and can be used to manipulate or affect the cell in some way.
FIGS. 1A-1C illustrate the change in ionic balance with respect to the change in polarization (e.g., the polarization illustrated in FIG. 1D). As illustrated in FIG. 1A, the cell is at resting potential, which can be established through the balance of K+ and Na+ ions in the intracellular and extracellular space. As illustrated in FIG. 1B, the cell adjusts the resting membrane potential through exchange of ions through specialized gates and channels (arrows 105) to achieve a different membrane potential to perform cellular functions. This can also occur in response to an externally imposed electric field. As illustrated in FIG. 1C, the cell is shown redistributing ions in the intra and extra cellular space to return to its resting potential.
When a small external electric field (e.g., less than about 500 mV) can be imposed on a cell, contingent on the magnitude of the field and positive or negative nature of the field, the cell can undergo hyperpolarization or depolarization. During the hyperpolarization, the transmembrane voltage can decrease to a larger negative value, and during depolarization the transmembrane voltage can increase to a positive value. When the external field can be removed, the cell can use ATP, ion pumps and voltage activated channels to return the cell back to its resting potential value. ATP can be consumed during this process, and energy consumed in the process of returning the cell to resting values can amount to up to about 25-30% of a cell's total metabolic energy consumption during this process. Additionally, it is previously known that the external field can cause the activation of electrically sensitive structures in the cell, such as the plasma membrane, and the voltage gated ion channels and pumps. These structures are also known to undergo mechanical deformation, but can rapidly return to normal shape when the external field can be removed. Finally, the external electric field can cause generation and flow of inward or outward currents. (See, e.g., FIG. 2). The flow of currents can be dictated by the sign of the external fields. These bio-electric phenomena are well documented in the literature, and can form the basis of experimental techniques (e.g., patch clamp studies) and clinical methods (e.g., neuromuscular activation). Such electrical stimulation of cells can be known to be safe and regularly used for therapeutic purposes.
It is previously known that deliberate application of an electric field in the form of discrete unipolar pulses, with a square or an exponential shape, can result in creation of nano-sized pores in the plasma membrane. This phenomenon is called electroporation or electro-permeabilization, and can be contingent on the selection of pulse parameters; the pores created in the plasma membrane can be enduring or transient. The former technique can be called irreversible electroporation (“IRE”), and the latter can be called reversible electroporation. Reversible electroporation can be used for the introduction of molecules or genetic material into the cells. The pulse parameters for reversible electroporation can be carefully chosen to cause the cell to survive, or remain viable, following the permeabilization process. In the case of irreversible electroporation, pulse parameters can be deliberately chosen that can result in creation of pores in cell membranes that can be permanent. Thus, the cells can be unable to recover from this process, and as a result, can undergo necrosis due to acute injury. A dramatic increase in electric impedance of the treated tissue can be observed, or can be considered as evidence of both reversible and irreversible forms of electroporation taking place in the targeted cells.
The application of irreversible electroporation can be enhanced by identifying square pulse parameters such as pulse width, applied voltage, number of pulses and pulse repetition time that can facilitate delivery of IRE while minimizing the collateral rise in tissue temperature due to the passage of electrical current. This enhanced technique has been termed non-thermal IRE. Non-thermal IRE can be achieved through application of pulse parameters such that a rise in transmembrane voltage of 0.7-1.0V can be achieved leading to permanent electroporation of the tissue without cell injury due to thermal mechanisms. While the field of IRE has been reviewed, its application can be enhanced by modifications that can minimize temperature related effects of the pulse application. Imaging techniques can be used, such as electrical impedance tomography that can specifically exploit the decrease in tissue impedance following non-thermal IRE to map ablated tissue in-vivo. Broadly, the electric field strengths of about 500-2500V/cm or about 800-1000V/cm can be used (e.g., such that a minimum sustained threshold of about 637V/cm) is sustained to achieve IRE in tissue. In addition, to achieve non-thermal IRE, the pulse width has to be multiple times longer than that of the membrane charging time of the cell types in the target tissue, the applied voltage and its derivative, and the electric field strength can be significantly larger than what can be used in reversible electroporation. The number of pulses used to achieve IRE can be larger than what can be used for reversible electroporation, and can be such that the inter pulse spacing or duration allows for the buildup of transmembrane voltage to the desired IRE threshold. (See, e.g., References 1-4).
Additional enhancements to IRE have been explored to exploit the temperature dependent properties of change in the transmembrane potential to achieve IRE at thresholds that can be lower than what has been reported by (see, e.g., References 1-4). This can be achieved by using a train of two different pulses, such that the first sequence can be used to heat the target tissue but can be insufficient to ablate the tissue by itself, and which can be followed by a second pulse sequence that can induce IRE in the targeted region. The heating caused by the first pulse sequence can be used to reduce the transmembrane voltage threshold of IRE from about 0.7-1.0V to about 0.5V. To achieve this effect, the first pulse sequence can be applied rapidly, with low external electric field strength and the second pulse sequence can be applied with pulse considerations such as a pulse length longer than membrane charging time. Inter-pulse spacing can be used to facilitate the building of transmembrane potential and voltage thresholds that can be sufficiently large to induce IRE. While this technique can reduce the threshold potential requirements for achieving IRE, it can no longer be considered a non-thermal ablation technique. The increase in temperature, while insufficient to ablate cells, can otherwise be sufficient to destroy heat sensitive structures such as bile ducts and nerves that can be in vicinity of the treatment regions. This reduces some of the benefits of non-thermal IRE. (See, e.g., References 1-7).
Another technique can use very high voltage pulsed electric fields to directly permeabilize the nuclear membrane of cells without affecting the plasma membrane. (See, e.g., Reference 8). This technique has been described as nanoporation or supraporation. In this technique, pulses of nanosecond width and spacing can be used in conjunction with field strengths larger than about 10,000V/cm. The pulses can have a width much shorter than the membrane charging time of most cells, and therefore, can bypass charging the plasma membrane instead of directly affecting membrane of intra-cellular organelles such as the nucleus and the mitochondria. At the end of the treatment application, the cells have been reported to undergo apoptosis due to disrupted nuclear architecture, but the plasma or cell membrane itself may not be permeabilized. Therefore, concomitant electrical conductivity changes seen in IRE can be absent here. Nanoporation is also believed to kill cells without large temperature changes and can be considered a non-thermal ablation technique that can otherwise be benign to non-cellular structures in the treated area.
Further enhancements to both non-thermal IRE and nanoporation can be achieved with a derivative technique called high frequency irreversible electroporation (“HI-FIRE”). The use of a pulsed waveform has been researched such that the pulses can be shorter than the membrane charging time of either the nuclear or the plasma membrane. (See, e.g., References 9-13). A multitude of such pulses, or significantly larger number than what can typically be used for either IRE or nanoporation, can be applied with a very short inter-pulse spacing. This arrangement can facilitate the spatial or temporal summation of the effect of these pulses, and effectively facilitates them to increase the transmembrane potential to achieve either nanoporation or IRE contingent on the applied field strength. A benefit of this technique can be that it can facilitate direction of ablation through layers such as epithelial cells or other cells with tight gap junctions that would otherwise get charged and ablated, therefore impeding the satisfactory ablation of an underlying target tissue. While the pulse parameters can be different from other techniques (see, e.g., References 1-7), this technique can use a similar range of field strengths and membrane potentials (e.g., about 0.7-1.0V) to achieve cell injury and death. (See, e.g., References 9-13). The high energy electrical fields used to induce IRE, the inability to selectively target cells, the concomitant tissue edema, the impact on electrical sensitive structures, and the effect of tissue heterogeneity on ablation outcomes can limit the application of IRE and associated techniques in the clinical setting.
A number of energy sources and associated ablation techniques can be clinically used for the therapy of patients. Exemplary ablation methods can be categorized on basis of the energy source used for causing injury to the cell. Ablation that predominantly uses temperature differences to cause cell injury can be called thermal ablation techniques which can include radiofrequency ablation, microwave ablation, cryoablation, some forms of electrocautery and laser therapy. Another category of ablation techniques can predominantly rely on application of strong external electric fields to cause pore formation in cell membranes. These techniques can be broadly termed electroporation. Electroporation may not be intended to cause permanent injury to the cell. Techniques such as irreversible electroporation, nano or supraporation, high frequency electroporation and enhanced electroporation can be derivative methods which can be meant for inducing cell death through permanent injury. These techniques can typically be called non-thermal techniques, as the primary cause of cell death may not be due to variations in tissue temperature. There can be other non-thermal ablation techniques, which include photodynamic therapy, argon-plasma coagulation, electrochemicaltherapy, electrochemotherapy and different forms of radiation therapy. In addition to these, many of these ablation techniques can be performed in combination with each other.
Thermal ablation techniques can use an applicator to provide an energy source or sink in proximity to the targeted region. Depending on the technique, the temperature gradient can be established through electromagnetic wave induced molecular friction or heating and/or cooling through adiabatic expansion of gases, joule heating processes or the application of energy through light sources. While there can be slight variations in the exact mechanism of cell death, generally cell necrosis can be induced through heat induced coagulation of proteins and direct thermal injury of cell components. In the case of cryoablation, cell death can be induced through creation of intra and extra-cellular ice crystals that can cause cell rupture and damage to interstitial tissue. Due to their working mechanism, there can be two fundamental shortcomings of these thermal ablation techniques. First, the technique can be non-targeted, destroying all forms of tissue, extra cellular matrix and other components that can fall within the region of altered temperature values. Because of this, scarring can be a common outcome following ablation and also otherwise healthy tissue, vasculature and critical structures such as nerves can also be permanently injured during treatment. Elevated temperatures can increase risk of perforation of lumen structures, and therefore can limit application of these ablation techniques in proximity to lumen such as the ureter, bile duct or the esophagus.
While some of these techniques have been adopted for mucosal ablation within lumen, their use can be limited to sub-millimeter depths. These techniques may be unable to treat tissue deeper than this depth without significant risk of perforating the tissue or inducing strictures in the long term. Some of these techniques have also been adopted for targeting nerves that can surround lumen, for example nerves surrounding the renal arteries or the bronchus. However, these ablations can be non-targeted in that they cannot selectively ablate the nerve without permanently injuring or destroying tissue that lies in the path of energy delivery. Therefore in these cases, thermal ablation can damage the muscularis and adventitia of the lumen supporting the nerves. The second significant shortcoming of thermal ablation techniques can be that they can be affected by heat sinks within the body. Perfusion and vascular flow can significantly affect the completeness and success with which these techniques can ablate cells within a target area. It may not be uncommon for failure of these techniques in ablating tissue adjacent to large blood vessels, which in fact can be a contra-indication for the use these techniques. (See, e.g., FIGS. 26A and 26B).
Thus, it may be beneficial to provide an exemplary embodiment of a system, method and computer-accessible medium for cell targeted in-vivo tissue ablation, without damaging surrounding non-targeted tissues, and which can overcome at least some of the deficiencies described herein above.